Attaining high activity with high selectivity at low temperature is challenging in the selective hydrogenation of phenol to cyclohexanone due to its high activation energy(E_a, 55–70 kJ/mol). Here we report a simple and efficient strategy for phenol hydrogenation catalyzed by Pd in aqueous phase at 30 °C by introducing air to promote the catalysis. With the assistance of air, >99% conversion and >99% selectivity were achieved over Pd(111)/Al_2O_3 with an overall turnover frequency(TOF) of621 h^(-1), ~80 times greater than that of the state-of-art Pd catalyst at 30 °C. Mechanism studies revealed that phenol was activated to generate phenoxyl radicals. The radicals were yielded from the reaction between phenol and hydroxyl radicals in the presence of hydrogen, oxygen and protic solvent on Pd. The phenoxyl pathway resulted in a low apparent E_a(8.2 kJ/mol) and thus high activity. More importantly, this strategy of activating substrate by air can be adapted to other Pd based catalysts, offering a new thinking for the rational design of cyclohexanone production in industry.
In wet chemical syntheses of noble metal nanocrystals,surfactants play crucial roles in regulating their morphology.To date,more attention has been paid to the effect of the surfactant on the surface energy of crystal facets,while less attention has been paid to its effect on the growth kinetics.In this paper,using the growth of Au-Pd alloy nanocrystals as an example,we demonstrate that different concentration of surfactant hexadecyltrimethyl ammonium chloride(CTAC)may cause the different packing density of CTA+bilayers on different sites(face,edge or vertex)of crystallite surface,which would change the crystal growth kinetics and result in preferential crystal growth along the edge or vertex of crystallites.The unique shape evolution from trisoctahedron to excavated rhombic dodecahedron and multipod structure for Au-Pd alloy nanocrystals was successfully achieved by simply adjusting the concentration of CTAC.These results help to understand the effect of surfactants on the shape evolution of nanocrystals and open up avenues to the rational synthesis of nanocrystals with the thermodynamically unfavorable morphologies.
表面结构决定了晶体材料的许多重要的物理化学性质.可控制备具有特定表面结构的微纳米晶体,从而实现对晶体材料性能的改善,引起了广泛的关注.本课题组近期相关的研究表明,晶体生长体系中,生长单元的过饱和度决定了晶体的表面结构.基于已有研究结果,本文通过调节混合溶剂体系中乙二醇和水的相对体积比及Na OH的用量,改变晶体生长体系中生长单元的过饱和度,合成了亚微米尺寸的Na Ta O3立方体、削角立方体、削角削棱立方体和准球体.低过饱和度条件下,形成的Na Ta O3颗粒是低能{100}晶面裸露的立方体;随过饱和度的增加,立方体的角和棱逐渐削去,裸露表面能更高的晶面;极高过饱和度条件下,则形成准球体Na Ta O3颗粒.这种准球体的形成难以用经典的晶体生长理论加以解释.通过研究同样受过饱和度影响的二维晶核生成功(Whkl),发现在极高的过饱和度的条件下,晶体生长趋于各向同性,合理地解释了准球体的形成机制.
Seed-mediated growth is the most general way to controllably synthesize bimetal nano-heterostructures.Despite successful instances through trial and error were reported, the way for second metal depositing on the seed, namely whether the symmetry of resulted nano-heterostructure follows the original crystal symmetry of seed metal, remains an unpredictable issue to date. In this work, we propose that the thermodynamic factor, i.e., the difference of equilibrium electrochemical potentials(corresponding to their Fermi levels) of two metals in the growth solution, plays a key role for the symmetry breaking of bimetal nano-heterostructures during the seed-mediated growth. As a proof-of-principle experiment, by reversing the relative position of Fermi levels of the Pd nanocube seeds and the second metal Au with changing the concentration of reductant(L-ascorbic acid) in the growth solution, the structure of as-prepared products successfully evolved from centrosymmetric Pd@Au core-shell trisoctahedra to asymmetric Pd-Au hetero-dimers. The idea was further demonstrated by the growth of Ag on the Pd seeds. The present work intends to reveal the origin of symmetry breaking in the seed-mediated growth of nano-heterostructures from the viewpoint of thermodynamics, and these new insights will in turn help to achieve rational construction of bimetal nano-heterostructures with specific functions.
